Abstract An essential objective for vaccines to protect against viruses that continually evolve to escape host immunity -- influenza and HIV being the most prominent human pathogens of current interest in this regard -- is to target selected, conserved epitopes and to understand and anticipate viral escape mechanisms. An underlying premise of this Program Project grant is that analysis of B-cell ontogeny following influenza-virus vaccination or infection can facilitate design of immunogens to elicit a broadly neutralizing response. We seek to answer critical questions about immune-response mechanisms, immunization strategies and B-cell immunogen design in quantitative terms and hence to provide a more secure structural and biophysical foundation for vaccine development. The proposed research strategy focuses on antibodies that recognize the receptor-binding site (RBS) on influenza HA -- one of two conserved sites shown to be broadly neutralizing antibody (bnAb) targets. We will combine analysis of human repertoires in response to influenza vaccination with similar analyses of repertoires following experimental immunizations of mice and rhesus macaques. We will test hypotheses concerning imprinting of the immune response by initial infection or vaccination, establish how reinfection or revaccination with HA presenting a mutated epitope influences broadening of the response through further affinity maturation, and integrate what we learn into tests in of proposals for immunogen design